Electric steelworks mainly use pressed scrap, sheets, galvanized elements and the like, which contain heavy metals such as Zn, Pb and Cd, which evaporate at a high temperature to which these furnaces operate (1600° C.), then condense as oxides when the smoke gets cold and are captured in sleeve filters. This dust is very fine and difficult to handle, its average chemical composition is shown in Table 1, where a 20-21% of Zn, 20-35% of Fe, 3 to 5% of Pb, 3% of chlorides, etc. are highlighted. Cd presence is only recorded in no more than 800 ppm.
TABLE 1Average Composition of steelworks dust (EAF dust), % p/pElementChileUSAFranceSpainFe34.428.521.825.9Zn20.019.021.218.6Pb4.682.13.63.63Cd0.076<0.01ND0.10Cr0.20.390.370.31As0.01-0.03NDNDNDCa1.911.85-10.012.83.50Ni0.0170.01-0.020.10-0.7Mo<0.01<0.02NDNDMn1.830.082.52.81Mg0.712.46-4.60ND1.53Cu0.2-030.77-2.930.250.54Si1.5 0.6-2.32ND1.65Cl3.51.35-2.491.753.43F—0.51-2.36NDNDK20.01-0.882.061.23Na1.370.06-1.122.231.27Al0.270.29-2.31ND0.44S1.08NDNDNDRefs. (4, 5)
When the Zinc content in the dust is at a level of 20 to 21%, it happens that a 20 to 30% of the same is found in the form of zinc ferrite, ZnFe2O4, that dissimilar to zinc oxide, ZnO, is hardly soluble in hydro-metallurgic treatments, both in acid and alkaline media, to which it is subjected to retrieve zinc.
Environmental regulation from almost any country in the World, which is increasingly rigorous, classifies the electric steelwork dust as a hazardous waste due to its content of heavy metals, which means that the waste can not be disposed without a previous treatment (5). On the other hand, the feasibility of recycling this dust to the same steelwork which produces it, to retrieve the iron it contains (25-35%), is difficult due to operational problems it causes because of the fineness of the dust and the presence of the above-mentioned contaminants. The hazardous character of steelworks dust is because of the presence of Pb, Cd and hexavalent Cr, which is measured by the Environmental Protection Agency of USA EPA SW-846 test, Method-1131 “TCLP—Toxicity Characteristic Leaching Procedure” (7,8). Table 2 shows lixiviation limits which wastes must meet.
TABLE 2Lixiviation limits (ppm) for EAF dustPbCdCrAsAgBaHgSeUUS EPA51555ND0.21NDCanada0.50.50.50.50.5500.010.1200Italy0.20.0220.50.5NDNDNDNDGermany2.00.510.01NDNDNDNDNDRefs. (7, 8 and 9)
In Chile and in most of Latin American countries, the above regulation is applied. Recently, environmental regulations have progressed around the world, for example, in the USA since 1988, the EPA has classified steelworks dust as dangerous (10) and in 1995 it was established that treatment technologies must comply with the TCLP test for 14 elements (11). Table 3 shows typical TCLP analysis for Chilean steelworks dusts, where it is observed that the national regulation for Cd is exceeded.
TABLE 3Typical results of TCLP Test for Chilean steelwork dusts, mg/LSamplePCdCrAAgBHgSe10.54.00.05<0.001<0.020.4<0.0010.1021.824.6<0.03<0.001<0.010.5<0.0010.14Norm515551000.21
Consequently, environmental considerations are key in the research and development of technologies to process this type of wastes. Thus, the strategy to follow should recover the valuable metals (Fe, Zn, Pb) and generate a no hazardous waste that can be stored without problems, or otherwise valorized through some other use.
Environmental considerations mean that the industrial processes to be applied should meet the environmental regulations established by each country, which are translated in rules and regulations expressed in the accomplishment of certain environmental standards regarding the quality of the air, emission of particulate matter and disposition of liquid and solid wastes.
For recycling, a number of processes are available nowadays. In the first place are the pyre-metallurgical processes represented by the Waelz process (12), which is the main technology in use of those called High Temperature Metal Recovery (HTMR). It is a robust and well established technology, having the following features:                1) It operates at high temperatures and makes an intensive use of energy.        2) Minimum economic size is of over 50,000 tons/year (13), which necessarily has led to set up regional plants in USA since the average dust generation in steelworks (EAF dust) is about 8,000 tons/year per plant (10). This faces the governmental pressure of restraining transportation of hazardous materials.        3) It generates low value products which require a hydrometallurgical treatment (washing) to be valorized.        4) It produces an inert slag which is used in roadways stabilization.        5) It releases important volumes of combustion gases into the atmosphere.        
Several other HTMR (10) technologies have been oriented to solve problems of the Waelz process, like installing small plants, processing dusts without having to agglomerate them, recycling iron and producing a no hazardous waste. Nevertheless, almost all of these processes also make an intensive use of energy and present problems in the condensation of volatile matter (Pb, Zn, Cd and salts as chlorides), reasons why many of them have been abandoned.
Hydro-metallurgical technologies are less developed, existing plants at pilot level or at small commercial scale. These technologies are becoming important due to the following reasons (14):                1) They provide environmental benefits as they do not generate air pollution.        2) They make possible the installation of much smaller plants, which in comparative terms need less investment and lower operational costs.        3) They have the ability to produce higher added value products (pure metals).        4) They have the ability to easily separate halures and provide an easy disposition of dumping.        5) They generate a solid recyclable to the EAF or HTMR.        
Problems still seeking solutions are the following (14, 5):                1) Low Zinc recovery because the Zn contained in the ferrite can not be dissolved in virtually any of the lixiviation media used, including mineral acids and strong basis.        2) Separation of metal is expensive.        3) The solid waste from the lixiviation stage it is not inert, what makes expensive its landfill dumping and also limits its applications.        
Research works are being performed with the process in alkaline media due to its advantages, especially, in front of the process in acid media (15). Advantages are:                1) Separation of iron in the lixiviation stage, since iron oxides are not soluble in alkaline media (16), different from zinc, lead and cadmium oxides, which are quickly dissolved. In acid media, an important amount of iron is dissolved, economically complicating its separation stages and its final disposition.        2) It produces a higher added value product (high quality zinc powder).        3) It is more favorable because of better handling and minimization of wastes (15).        
Improvements investigated in the alkaline process are referred to the release of zinc from the ferrite by fusion with soda and later lixiviation, with a recovery of a 95% (17) and lixiviation with soda through micro-waves (4), thus increasing zinc recovery in 5 to 10% in relation to the conventional system.
In short, from the point of view of environmental sustainability, hydro-metallurgical processes are better than the pyre-metallurgical ones, and within the hydro-metallurgical processes, the alkaline process has clear advantages over the acid media process, that is why then the importance this process is having nowadays, in spite of the fact that up to date no successful industrial realizations are known.
The first and main patents that formalize the alkaline hydro-metallurgical process (REZEDA procedure) are U.S. Pat. No. 3,326,783 and FR 2,510,141. The former is applied to the calcine coming from toasting of oxidized zinc minerals and the latter is applied to electric steelworks dusts.
In the 1970's, Amax Co. of New Jersey, USA, operated a pilot plant based on the alkaline process. Development of the same was abandoned due to technical and economic problems. An industrial plant was built years later in France, also on the basis of the alkaline process, and it was also abandoned.
Improvements tried to economically solve the elimination of chlorides and sulfates present in the steelworks dusts, whose concentration levels are limited by restrictions imposed in the electro-deposition stage.
Below, and in order to have a better understanding of the background, it is described the basic process (REZEDA procedure) and, afterwards, the improvements incorporated by others.
The basic hydro-metallurgical process in alkaline media, described in the above patents, includes a lixiviation stage of steelworks dust in soda solution, followed by filtration in which the solid waste with high iron content is separated after washing. Filtrate is purified through cementation with zinc powder, precipitating lead, cadmium, copper and other heavy metals, which are separated by filtration. Filtrate then goes to an electro-deposition stage, where the zinc powder is deposited and then filtered, washed and dried to obtain the dry powder. The stripped solution from the electro-deposition stage is recirculated to the lixiviation stage, previous elimination of saline impurities by evaporation. The stripped solution recirculating in a closed circuit is progressively enriched in saline impurities, specifically chlorides and sulfates which are added by the feeding of steelworks dusts to be treated, like soda and water, among others. These impurities must be eliminated since a determined concentration level of them must be maintained in the electrowinning stage. These saline impurities are eliminated like brine obtained through evaporation of part of the recirculating solution.
U.S. Pat. No. 3,326,783 does not provide information on the quality of the zinc powder obtained, or the waste disposition, neither as how saline impurities and washing waters are disposed of, important things in the economy of the process, which is explained because, by that time, environmental regulations were rather elemental.
FR 2,510,141 provides more precise details on the lixiviation, cementation and electrowinning stages. It claims a two stage lixiviation to make soluble the zinc present in both oxides and ferrites. Nevertheless, it indicates that just lixiviation of oxides is affordable and proposes the evaporation stage to eliminate salts. Regarding lixiviation waste, it only indicates that this is sterile, without presenting evidence or mentioning how it is disposed of. Regarding the quality of the zinc powder obtained, it only refers to the particle size distribution, wherein 82% is between 200 and 325 micron. It does not mention the impurities it contains or its metallic content, which impacts the profitability of the process.
FR 2,757,540 proposes to submit steelworks dust to a lixiviation process with aqueous bleach to eliminate chlorides and sulfates by dissolution and then subjecting the washed waste thus obtained to an alkaline lixiviation, cementation of the resulting alkaline solution and electrowinning of Zn from it to, finally, recirculate the stripped solution directly to the lixiviation step. On the other side, a fraction of near 70% of the washing water, charged with chlorides and sulfates, is recirculated to the washing stage with bleach, while the rest is submitted to a partial neutralization with acid to precipitate mainly lead hydroxide, which is sent to the alkaline lixiviation together with the wet waste produced by washing with bleach. The filtrate is treated in an evaporator to separate brine and water which is returned to the process. An analysis is as follows:                1) The desalinization pre-treatment of steelworks dusts consists of a process line (L1) autonomous and independent from a line (L2) of alkaline treatment itself, only tied because the latter is fed with both wet waste and precipitated hydroxides generated by the former, thus losing the degree of separation they already had.        2) The washing waters treated in the evaporator are only those generated in L1. Nevertheless, in L2 the following situation is present: the wet waste fed to this line carries a content of chlorides not over 0.03% (18) and a moisture content estimated between 10 and 30%, a level that depends on the kind of solid-liquid separation employed. Since the process line operates with a lixiviating solution in a closed circuit, there will be a cumulative effect of salts and water. The salt content is limited by the concentration imposed in the electrowinning stage and, in the same manner, the water content is limited by the soda concentration in the recirculating solution which should be kept around 300 to 400 g/L. On the other hand, in addition, it must considered that the washing waters of the different filtration stages in the process line get soda and zincate which must be recovered, making it necessary to have continuous or periodic purges and treat the washing waters in an evaporator. Nevertheless, the process recirculates to the leaching stage only a fraction of the solution that comes out of electrowinning, and neutralizes the rest with external acid spills coming from operations of galvanization. The end result is that the evaporation is eliminated, but soda is lost and liquid wastes are generated.        
Finally, FR 2,770,229 refers to conditioning of zinc powder obtained by electrowinning in an alkaline medium. Basically, the pulp containing zinc powder from electrowinning is filtered in an inert atmosphere. Washing of the Zn cake soaked in zincate solution is performed in demineralized and deoxygenated water, drying the Zn powder under vacuum (65 mm of Hg) at low temperature (20-30° C.). These operations are performed with just one equipment. It sets forth the use of ultrasound to accelerate the washing operation or to improve a quicker segregation of the powder. Zinc obtained is claimed to have a total zinc content of 99.6%, out of which 97-98% is metallic zinc and 2 to 3% is zinc oxide, and exhibits a particle size on the order of 6 microns.
The traditional processes described in all the above analyzed patents use conventional electrodeposition cells of rectangular design, whose surface is open to the atmosphere, amenable to alkaline fog formation and dissemination due to the rupture of hydrogen and oxygen bubbles. In addition, its hydrodynamics causes the production of Zn in the form of flakes which tend to adhere to the cathode surfaces, thus making necessary the use of complex vibration or sweeping systems.